This invention is in the field of auditory assessment and relates to the identification and evaluation of hearing impairment. More particularly, the invention describes a system and method for objectively evaluating an individual""s hearing abilities by recording auditory steady-state responses.
Hearing-impairment is a significant health problem, particularly at the two ends of the human life span. Approximately one in a thousand newborn infants and more than a quarter of adults over the age of 65 have a significant hearing loss. In the case of infants, early detection of hearing loss is necessary to ensure that appropriate treatment is provided at an early stage and that the infant can develop normal speech and language. The detection of a hearing-impairment requires a measurement of how well someone can hear sound (i.e. audiometry).
Conventional audiometry is performed by having a subject respond to acoustic stimuli by pressing a button, saying xe2x80x9cyesxe2x80x9d, or repeating words that may be presented in the stimulus. These tests are subjective in nature. Audiometry allows an audiologist to determine the auditory threshold of the subject, which is defined as the lowest intensity at which a sound can be heard. The audiologist evaluates the auditory threshold of a subject by using a stimulus that most commonly consists of a pure tone. The stimulus is presented via earphones, headphones, free field speakers or bone conduction transducers. The results are presented as an audiogram which shows auditory thresholds for tones of different frequencies. The audiogram is helpful for diagnosing the type of hearing loss a subject may have. The audiogram can also be used to fit a hearing aid and adjust the level of amplification of the hearing aid for subjects who require hearing aids.
Audiometry may also involve subjective testing at supra-threshold intensities to determine how well the subject""s auditory system discriminates between different sounds (such as speech) presented at intensities at which they normally occur. The audiologist will therefore determine how many simple words a subject can accurately recognize at different intensities with and without different amounts of background noise. The audiologist may also conduct tests which measure how well the subject can discriminate changes in the intensity or frequency of a sound or how rapidly these changes occur.
Conventional audiometry cannot be performed if the subject is an infant, young child or cognitively impaired adult. In these cases, objective tests of hearing are necessary in which the subject does not have to make a conscious response. Objective audiometry is essential for detecting hearing impairment in infants or elderly patients as well as for evaluating functional hearing losses. Furthermore, few objective tests have been developed for supra-threshold tests of speech, frequency, or intensity discrimination.
One form of objective audiometry uses auditory evoked potentials. Auditory evoked potential testing consists of presenting the subject with an acoustic stimulus and simultaneously or concurrently sensing (i.e. recording) potentials from the subject. The sensed potentials are the subject""s electroencephalogram (EEG) which contain the subject""s response to the stimulus if the subject""s auditory system has processed the stimulus. These potentials are analyzed to determine whether they contain a response to the acoustic stimulus or not. Auditory evoked potentials have been used to determine auditory thresholds and hearing at specific frequencies.
One particular class of auditory evoked potentials is steady-state evoked potentials (SSAEPs). The stimulus for the SSAEP consists of a carrier signal, which is usually a sinusoid, that is amplitude modulated by a modulation signal which is also usually a sinusoid. The SSAEP stimulus is presented to the subject while simultaneously recording the subject""s EEG. If the auditory system of the subject responded to the SSAEP stimulus, then a corresponding steady-state sinusoidal signal should exist in the recorded EEG. The signal should have a frequency that is the same as the frequency of the modulation signal (i.e. modulation frequency). The presence of such a corresponding signal in the EEG is indicative of a response to the SSAEP stimulus. Alternatively, the phase of the carrier signal may be frequency modulated instead of or in addition to amplitude modulation to create the SSAEP stimulus.
The SSAEP stimulus is sufficiently frequency-specific to allow a particular part of the auditory system to be tested. Furthermore, the SSAEP stimulus is less liable to be affected by distortion in free-field speakers or hearing aids. Typical modulation frequencies which are used in SSAEP stimuli are between 30 to 50 Hz or 75 to 110 Hz. The latter range may be particularly useful for audiometry because at these rates, the SSAEP responses are not significantly affected by sleep and can be reliably recorded in infants. Furthermore, SSAEP responses at these rates result in audiometric threshold estimates that are well correlated with behavioral thresholds to pure tone stimuli. In SSAEP testing, the presence or absence of an SSAEP response to an SSAEP stimulus can be determined using several statistical techniques.
However, objective audiometry employing SSAEP testing is time-consuming because the amplitude of the SSAEP response is quite small compared to the background noise which is the subject""s ongoing brain activity (i.e. EEG) while the test is being conducted. The SSAEP response thus has a small signal-to-noise ratio (SNR) which makes it difficult to detect the SSAEP response in a short time period. One technique to reduce SSAEP testing time is to use a multiple SSAEP stimulus which combines several SSAEP test signals (i.e. where a test signal is meant to mean one SSAEP stimulus). The potentials sensed from the subject during the presentation of the multiple SSAEP stimulus contains a linear superposition of SSAEP responses to each SSAEP test signal in the multiple SSAEP stimulus. This makes it possible to record the SSAEP responses to multiple (e.g., four or eight) stimuli in the same time that it takes to record the response to a single stimulus. Therefore, this technique results in a reduction of test time since the SSAEP responses to several SSAEP test signals may be detected concurrently. However, the SNR for each SSAEP response is still small and the testing time for recording the response to a single SSAEP stimulus has not been reduced. To reduce the SSAEP test time techniques are required to either increase the amplitude of the SSAEP response and/or decrease the amplitude of the noise that is recorded along with the SSAEP response. A more sensitive statistical method that can detect SSAEP responses with small SNRs would also be useful.
While objective testing identifies that a subject has a hearing loss, the next step is usually to treat the subject by providing them with a hearing aid. However, if the subject is an infant, a method is required to objectively adjust the hearing aid since this cannot be done with conventional subjective methods. Some objective methods have been developed such as determining the real-ear insertion gain when a hearing aid is in place. However, this method is only useful if one knows the actual unaided audiometric thresholds of the subject so that the hearing aid can be adjusted to match prescriptive targets. Furthermore, placement of a probe-tube in an infant can be challenging. There have also been methods based on click evoked auditory evoked potentials (i.e. wave V of the click-evoked ABR) but the stimuli used in these methods are restricted to certain frequency ranges and do not test the ability of the hearing aid to process continuous signals like speech. Accordingly, there still remains a need for an objective method to measure the benefits of a hearing aid in patients where behavioral thresholds and real-ear measurements are difficult to obtain.
The present invention is an apparatus for recording SSAEP responses and a set of methods for using the apparatus to test various aspects of a subject""s auditory system. The apparatus comprises hardware to present SSAEP stimuli, to acquire EEG data while simultaneously presenting the SSAEP stimuli, and to analyze the EEG data to detect the presence of SSAEP responses. The apparatus further comprises software to enable the creation and presentation of the SSAEP stimuli, the acquisition of the EEG data (i.e. electrophysiological potentials) and the analysis of the EEG data. The software further enables displaying the results of ongoing testing and the final results of the test as well as the storage of the test results for subsequent viewing and/or analysis.
The present invention also includes software adapted to effect noise reduction algorithms which may include sample weighted averaging. The software is also adapted to effect statistical tests that are used to detect the SSAEP responses to the SSAEP stimuli. These statistical tests may include the phase weighted t-test, the phase zone technique and the modified Rayleigh test of circular uniformity (MRC).
The present invention also uses particular types of SSAEP stimuli to increase the amplitude of the resulting SSAEP responses. These SSAEP stimuli may include a combined amplitude modulation and frequency modulation signal in which the phase of the frequency modulated signal is adjusted relative to the phase of the amplitude modulated. These SSAEP stimuli may also include using an exponential modulation signal. The present invention also uses an SSAEP stimulus consisting of an independent amplitude modulation signal and frequency modulation signal wherein the AM modulation rate is different than the FM modulation rate. This stimulus evokes two SSAEP responses that can be independently analyzed.
In another aspect of the invention, these SSAEP stimuli can be used for a variety of objective tests such as determining audiometric thresholds and testing the aided and unaided hearing of a subject. The present invention further comprises several other audiometric protocols including latency tests, AM/FM discrimination tests, rate sensitivity tests, aided hearing tests, depth sensitivity tests and supra-threshold tests.
The present invention further comprises databases of normative data which can be used to construct SSAEP stimuli, detect SSAEP responses and determine whether detected SSAEP responses are indicative of normal or abnormal hearing. The databases contain data which are grouped by subject characteristics such as age, sex and state over a variety of stimulus characteristics such as type of SSAEP stimulus, the type of modulation (amplitude versus frequency), the modulation rates and modulation depth. The database also preferably contains data about SSAEP response characteristics such as latency and ratio of amplitudes of SSAEP responses to amplitude modulated and frequency modulated SSAEP stimuli.
In an alternative embodiment, the apparatus is adapted to perform multi-modality testing in which more than one sensory modality (i.e. vision and audition) of the subject is tested simultaneously.
The invention comprises a method of testing the hearing of a subject comprising the steps of:
(a) selecting at least one test signal;
(b) modulating at least one of the amplitude and frequency of the at least one test signal by an exponential modulation signal to produce at least one modulated test signal;
(c) transducing the at least one modulated test signal to create an acoustic stimulus and presenting the acoustic stimulus to the subject;
(d) sensing potentials from the subject while substantially simultaneously presenting the acoustic stimulus to the subject; and,
(e) analyzing the potentials to determine whether the potentials comprise data indicative of the presence of at least one steady-state response to the acoustic stimulus.
The invention further comprises testing the hearing of a subject comprising the steps of:
(a) creating an optimum-vector mixed modulation test signal comprising at least one signal having an amplitude modulated component with a first phase and a frequency modulated component with a second phase wherein the second phase is adjusted relative to the first phase to evoke an increased response from the subject;
(b) transducing the test signal to create an acoustic stimulus and presenting the acoustic stimulus to the subject;
(c) sensing potentials from the subject while substantially simultaneously presenting the acoustic stimulus to the subject; and,
(d) analyzing the potentials to determine whether the potentials comprise data indicative of the presence of at least one steady-state response to the acoustic stimulus.
In another aspect, the invention comprises a method for testing the hearing of a subject comprising the steps of:
(a) creating a test signal comprising at least one independent amplitude modulated and frequency modulated signal having an amplitude modulated component and a frequency modulated component, wherein the amplitude modulated component comprises a first modulation frequency and a first carrier frequency and the frequency modulated component comprises a second modulation frequency and a second carrier frequency wherein the first modulation frequency is substantially different from the second modulation frequency and the first carrier frequency is substantially similar to the second carrier frequency;
(b) transducing the test signal to create an acoustic stimulus and presenting the acoustic stimulus to the subject;
(c) sensing potentials from the subject while substantially simultaneously presenting the acoustic stimulus to the subject; and,
(d) analyzing the potentials to determine whether the potentials comprise data indicative of a steady-state response to each amplitude modulated component and a steady-state response to each frequency modulated component.
In another aspect, the invention comprises an apparatus for testing the hearing of a subject, wherein the apparatus comprises:
(a) a signal creator adapted to create a test signal comprising at least one combined amplitude modulated and frequency modulated signal having an amplitude modulated component with a first phase and a frequency modulated component with a second phase wherein the signal creator comprises means for adjusting the second phase relative to the first phase;
(b) a transducer electrically coupled to the processor and adapted to transduce the test signal to create an acoustic stimulus and present the acoustic stimulus to the subject;
(c) a sensor adapted to sense potentials from the subject while the acoustic stimulus is substantially simultaneously presented to the subject; and,
(d) a processor electrically coupled to the sensor and adapted to receive the potentials and analyze the potentials to determine if the potentials comprise data indicative of at least one response to the acoustic stimulus.
In another aspect, the invention comprises a method of analyzing potentials to determine whether the potentials comprise data indicative of the presence of at least one steady-state response to a steady-state evoked potential stimulus. The method comprises the steps of:
(a) presenting an evoked potential stimulus to a subject;
(b) sensing potentials from the subject while substantially simultaneously presenting the stimulus to the subject to obtain a plurality of data points;
(c) transforming the plurality of data points into a second plurality of data points;
(d) biasing the second plurality of data points with an expected phase value to obtain a plurality of biased data points; and,
(e) applying a statistical test to the plurality of biased data points to detect the response.
The invention further comprises a method of detecting a response to an evoked potential stimulus comprising the steps of:
(a) presenting an evoked potential stimulus to a subject;
(b) sensing potentials from the subject while substantially simultaneously presenting the stimulus to the subject to obtain a plurality of data points; and,
(c) calculating phase values for the plurality of data points,
wherein, a response is detected if an adequate number of the calculated phase values fall within a predetermined phase value range.
The invention further comprises an apparatus for testing the hearing of a subject, wherein the apparatus comprises:
(a) a signal creator adapted to create a test signal;
(b) a transducer electrically coupled to the signal creator and adapted to transduce the test signal to create an acoustic stimulus and present the acoustic stimulus to the subject;
(c) a sensor adapted to sense potentials from the subject while the acoustic stimulus is substantially simultaneously presented to the subject; and,
(d) a processor electrically coupled to the sensor and adapted to receive the potentials and analyze the potentials to determine if the potentials comprise data indicative of at least one response to the acoustic stimulus;
wherein the analysis involves biasing the potentials based on an expected phase value. The apparatus further comprises a database of expected phase value data correlated to subject characteristics and stimulus characteristics.
In yet another aspect, the invention comprises a method of noise reduction for a plurality of data points which are obtained during steady-state evoked potential testing, wherein the plurality of data points comprise at least one signal and noise and wherein said method comprises the steps of:
(a) obtaining said plurality of data points;
(b) separating said plurality of data points into a plurality of epochs; and,
(c) applying an adaptive noise reduction method to each epoch.
In another aspect, the invention comprises a method of objectively testing the hearing of a subject comprising the steps of:
(a) selecting an auditory test;
(b) creating an appropriate test signal comprising at least one component for the auditory test;
(c) transducing the test signal to create a stimulus and presenting said stimulus to the subject;
(d) sensing potentials from the subject while substantially simultaneously presenting the stimulus to the subject; and,
(e) analyzing the potentials to detect at least one response.
In another aspect, the invention further comprises an apparatus for objectively testing the hearing of a subject, wherein the apparatus comprises:
(a) a selector adapted for selecting an auditory test to perform on the subject;
(b) a signal creator electrically coupled to the selector and adapted to create an appropriate test signal comprising at least one component for the test;
(c) a transducer electrically coupled to the signal creator and adapted to transduce the test signal to create an acoustic stimulus and present the acoustic stimulus to the subject;
(d) a sensor adapted to sense potentials from the subject while the acoustic stimulus is substantially simultaneously presented to the subject;
(e) a processor electrically coupled to the sensor and adapted to receive the potentials and analyze the potentials to determine if the potentials comprise data indicative of at least one response to the acoustic stimulus; and,
(f) a programmable hearing aid coupled to said processor, wherein, the programmable hearing aid comprises a plurality of programmable gain factors for different frequency regions and at least one programmable filter slope.
In an alternative embodiment, the invention comprises a method of testing at least two senses of a subject, wherein the method comprises the steps of:
(a) selecting a first steady-state test signal to test a first sensory modality;
(b) transducing the first steady-state test signal to create a first stimulus and presenting the first stimulus to the subject;
(c) selecting a second steady-state test signal to test a second sensory modality;
(d) transducing the second steady-state test signal to create a second stimulus and presenting the second stimulus to the subject;
(e) sensing potentials while substantially simultaneously presenting both stimuli to the subject; and,
(f) analyzing the potentials to determine whether the potentials comprise data indicative of at least one steady-state response to the stimuli.
In an alternative embodiment, the invention further comprises an apparatus for testing at least two senses of a subject, wherein the apparatus comprises:
(a) a signal creator adapted to create a first steady-state test signal and a second steady-state test signal;
(b) a first transducer electrically coupled to the selector and adapted to transduce the first test signal to create a first stimulus and present the first stimulus to the subject;
(c) a second transducer electrically coupled to the selector and adapted to transduce the second test signal to create a second stimulus and present the second stimulus to the subject;
(d) a first sensor adapted to sense first potentials from the subject while the first stimulus is substantially simultaneously presented to the subject;
(e) a second sensor adapted to sense second potentials from the subject while the second stimulus is substantially simultaneously presented to the subject;
(f) a processor electrically coupled to the first sensor and adapted to receive the first potentials and analyze the first potentials to determine if the first potentials comprise data indicative of at least one response to the first stimulus; and,
(g) the processor, electrically coupled to the second sensor and adapted to receive the second potentials and analyze the second potentials to determine if the second potentials comprise data indicative of at least one response to the second stimulus,
wherein, each stimulus is presented substantially simultaneously.
Further objects and advantages of the invention will appear from the following description, taken together with the accompanying drawings.